The utilization of rotary wing aircraft or helicopters is increasing. Helicopters have capabilities which are uniquely suited to the performance of some missions, for which fixed wing aircraft would be unsuitable. Helicopters require minimum space for takeoff and landing, and are capable of hovering. On the other hand, fixed wing aircraft are generally capable of higher forward speeds, but require lengthy runways for takeoff and landing and are not capable of hovering. The helicopter is highly maneuverable, particularly at low speeds, and is an ideal vehicle for observation or point to point transportation of personnel or cargo.
In some instances, it is desirable to locate instrumentation or other components above the lifting rotor of a helicopter. For instance, it can be advantageous during military operations to mount surveillance equipment or sighting devices above the rotor. This allows the helicopter to hover in a concealed position with minimum exposure to enemy detection and fire.
Above-rotor mounting of such devices, however, involves several difficult problems. Such devices must be connected to the rotor, the origin of large vibratory forces which can adversely affect the operation and useful life of any device attached thereto. Sighting devices and other electronic instruments are relatively delicate and have structural design limitations with respect to translational and rotational motions. In addition, the kinematics of the mounted device are very important. Operational accuracy of such devices is affected in various degrees by the particular motions thereof. Fore and aft movement of a sighting device, for instance, is less critical than roll and pitch. The prior art attempts to provide a suitable mounting have allowed excessive rotation and/or vibration of the mounted device, and reduced mobility of the rotor hub. For some rotor designs, this reduced mobility results in higher blade loads and increased hub excitation forces and vibrations. A further consideration for some helicopter designs is to mount the equipment above the rotor in such a manner that vibrations in the fuselage, which affect pilot proficiency, crew and passenger comfort, and equipment reliability, are not seriously degraded. Some mounts require very soft mounting frequencies to achieve the desired isolation, which is accompanied by undesireable low frequency transients. There is thus a need for a stiffer, more stable above-rotor mount which does not adversely affect mobility of the rotor hub.